Hydroxycarboxylic acids and hydroxycarboxylic acid salts have been described as chelating agents capable of sequestering metal ions in solution (Mehltretter, 1953; Abbadi, 1999). Hydroxycarboxylic acid salts as sequestering agents for metal ions such as calcium and magnesium, in general perform poorly compared to common sequestering agents such as sodium tripolyphosphate (STPP), ethylenediaminetetraacetate (EDTA), or nitrilotriacetate (NTA). In spite of low sequestering capacity, hydroxycarboxylic acid salts are of interest because they are typically biodegradable, non-toxic, and derived from renewable resources such as carbohydrates. Therefore, the use of hydroxycarboxylic acid salts as replacement sequestering agents for STPP and EDTA is advantageous, especially in applications where the compounds may be discharged into the environment. The performance of hydroxycarboxylic acid salts as sequestering agents for hard water ions can be boosted by the addition of suitable oxoacid anion compounds such as borate and aluminate. The boost in performance arises from the formation of diester complexes between the two adjacent hydroxyl groups of the hydroxycarboxylic acid salt and the borate or aluminate as described by van Duin et al (Carb. Res. 1987, 162, 65-78 and J. Chem. Soc. Dalton Trans. 1987, 8, 2051-2057). The work of van Duin et al. shows that diester complex formation occurs with compounds containing two vicinal hydroxyl groups, preferably in the threo configuration. The stability of the complexes is pH dependent with improved stability coming at higher pHs. Complexes between salts of hydroxycarboxylic acids and either sodium borate or sodium aluminate have been described as calcium sequestering agents for use in detergent applications (Hessen, U.S. Pat. No. 4,000,083; Tumerman, U.S. Pat. No. 3,798,168; and Miralles et al., U.S. Pat. No. 8,153,573). Therefore it is well known that complexes between salts of polyhydroxycarboxylic acids and suitable oxoacid anion salts such as sodium aluminate and sodium borate are useful as divalent metal ion sequestering agents for use in applications such as detergents. Surprisingly, we have found that the calcium sequestering performance of the complexes between salts of polyhydroxycarboxylic acids and suitable oxoacid anion salts can be improved by the addition of certain sequestering agents such as citrate salts. This is unexpected considering that performance of citrate is not improved by the addition of sodium aluminate or sodium borate as shown by van Duin et al. (Carb. Res. 1987, 162, 65-78).
Many chemical compounds that have traditionally been used as metal sequestering agents are phosphorus based. Through environmental regulations, the use of phosphorus compounds in applications where the material is discharged into surface water continues to be restricted. These regulations have created a need for environmentally acceptable materials for use as metal sequestering agents for a variety of applications.
One application in which metal sequestering agents are useful is in detergent formulations. Detergents are cleaning mixtures composed primarily of surfactants, builders, bleaching-agents, enzymes, and fillers. Two of the major components are surfactants and builders. The surfactants are responsible for emulsification of oil and grease while builders are added to extend or improve the cleaning properties of the surfactant. The builder can be a single substance or a mixture of substances and commonly serve multiple functions. An important builder function is the sequestration of metal cations, typically calcium and magnesium cations in hard water. The builders act as water softening agents by sequestering calcium and magnesium cations and thus prevent the formation of water insoluble salts between the cations and anion components in the wash solution, such as surfactants and carbonate. In the case of laundry detergents, builders also help prevent the cations from binding to cotton, a major cause of soil retention on cotton fabrics. Other functions of builders include increasing alkalinity of detergent solutions, deflocculating surfactant micelles, and inhibiting corrosion.
The first builders used in commercial detergents were phosphate salts and phosphate salt derivatives. Sodium tripolyphosphate (STPP) was, at one time, the most common builder in both consumer and industrial detergents. Phosphate builders were also touted as corrosion inhibitors for the metal surfaces of washing machines and dishwashers. Phosphates have been gradually phased out of detergents over the past 40 years primarily due to environmental concerns regarding discharge of phosphate rich waste water into surface waters giving rise to eutrophication and ultimately hypoxia (Lowe, 1978). High performance replacements for phosphates in detergents are still sought after.
Conventional detergents used in the vehicle care, food and beverage (e.g., the dairy, cheese, sugar, meat, food, and brewery and other beverage industries), warewashing and laundry industries include alkaline detergents. Alkaline detergents, particularly those intended for institutional and commercial use, generally contain phosphates, nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA). Phosphates, NTA and EDTA are components commonly used in detergents to aid in soil removal and to sequester metal ions such as calcium, magnesium and iron.
In particular, NTA, EDTA or polyphosphates such as sodium tripolyphosphate and their salts are used in detergents because of their ability to solubilize preexisting inorganic salts and/or soils. When calcium, magnesium and iron salts precipitate, the crystals may attach to the surface being cleaned and cause undesirable effects. For example, calcium carbonate precipitation on the surface of ware can negatively impact the aesthetic appearance of the ware, giving an unclean look. In the laundering area, if calcium carbonate precipitates and attaches onto the surface of fabric, the crystals may leave the fabric feeling hard and rough to the touch. In the food and beverage industry, the calcium carbonate residue can affect the acidity levels of foods. The ability of NTA, EDTA and polyphosphates to remove metal ions facilitates the detergency of the solution by preventing hardness precipitation, assisting in soil removal and/or preventing soil redeposition into the wash solution or wash water.
While effective, phosphates and NTA are subject to government regulations due to environmental and health concerns. Although EDTA is not currently regulated, it is believed that government regulations may be implemented due to environmental persistence. There is therefore a need in the art for an alternative, and preferably environment friendly, cleaning composition that can replace the properties of phosphorous-containing compounds such as phosphates, phosphonates, phosphites, and acrylic phosphinate polymers, as well as non aminocarboxylates such as NTA and EDTA.